1. Field of the Invention
This invention is directed to providing a means to extend the high temperature corrosion life of gas turbine blades.
2. Brief Description of the Prior Art
Gas turbine engines are used for power generation in many instances. They are used in marine environments also and therein the blades and vanes suffer a form of material degradation known as hot corrosion. The hot corrosion process involves the interaction of the metal blade and vane surfaces with the sulfur dioxide and sea salt present in the combustion gases within the engine. The solution to this problem has been to develop special metallic coatings to extend the services life of these parts. These coatings are often a mixture of cobalt, chromium, aluminum and yttrium. These coatings are identified as a class by the acronym CoCrAlY.
The hot corrosion process in a marine environment has been designated type 2 or low-temperature hot corrosion. The major parameters of the hot corrosion process are: sulfur in the fuel forms sulfur dioxide during combustion, sulfur dioxide reacts with the cobalt in the turbine blade coating and forms cobalt sulfate, sodium sulfate from the sea salt ingested by the engine deposits in solid form on the turbine blades; and, in the temperature range of about 677.degree. C. to about 732.degree. C., the operating temperature of the engine in a marine environment, the sodium sulfate and the cobalt sulfate form a mixed salt that has a lower melting point than either of the pure salts. This lower melting point of this mixed salt results in the degradation of the coating.
The hot corrosion process can be stopped by preventing the formation of cobalt sulfate. The aluminum scale that forms on these coatings should do just that. However, it is now known that the aluminum scales on these coatings contain yttrium and the yttria-rich phases in these oxide scales provides initiation sites for the hot corrosion process. This invention avoids these problems and enhances the hot corrosion process life by putting a ceramic layer onto the outer surface of the CoCrAlY which is not easily defeated by the hot corrosion process.